1. Field of the Invention
This invention relates generally to the fabrication of semiconductor devices and more specifically to a method for reducing the dielectric constant of materials used to separate conducting leads and structures in those devices.
2. Description of the Related Art
As the size of integrated microelectronic circuits continues to shrink, conducting structures and leads fabricated within those circuits must be positioned in closer proximity to each other both horizontally and vertically. This introduces the problem of intra-wall capacitative coupling between those structures and leads, which both adversely affects the time dependence of electrical signals (eg by producing increased RC delay in metallic interconnects) and produces crosstalk between nearby portions of the circuit. One way to reduce the capacitative coupling is to lower the dielectric constant of the material used to separate the conducting leads and structures from each other. The dielectric material most commonly used to separate these leads and structures is silicon dioxide, which has a dielectric constant of approximately 3.9. Vacuum, which is the ideal, perfect dielectric, has a dielectric constant of exactly unity (1.000), while air, at atmospheric pressure, has a dielectric constant of 1.00059.
The predominant present art utilizes silicon dioxide dielectrics, deposited either by chemical vapor deposition (CVD-oxide) or by means of plasma assisted deposition (HDP-oxide). These dielectrics have a resulting dielectric constant of between 3 and 4.
In an alternative form of the present art, dielectrics with lower dielectric constants than that of silicon dioxide are deposited. These dielectrics are typically spin-coated or CVD deposited on a thin liner of silicon dioxide that is first deposited over the conducting structures. Examples of this present practice include spin-coating of a non-porous polymer (eg. FLARE, a proprietary material having a dielectric constant between 2.5 and 3.0, manufactured by Allied Signal Corp.). Alternatively, various porous materials can be spin-coated (eg. Xerogel or Nanofoam, manufactured by Allied Signal Corp.), attaining lower dielectric constants, which still exceed 1.5. Other materials can be deposited by CVD (eg. Black Diamond, manufactured by Applied Materials Corp. and Coral, manufactured by Novellus Corp.), but these still have dielectric constants of between 2.5 and 3.
More recent attempts to reduce the dielectric constant have produced methods that introduce air into the gap-filling dielectric material or totally replace the gap-filling material with air. One such method (Havemann et al., U.S. Pat. No. 5,461,003) involves deposition of a disposable solid material as a temporary gap-fill, covering said material with a porous layer, then dissolving the material and allowing it to escape through the porous layer, leaving behind air-filled gaps. Another method (Gnade et al., U.S. Pat. No. 5,750,415) involves the deposition of a disposable liquid over conducting leads. A porous film of silica is deposited over the liquid and then gelled, to lower its porosity. The disposable liquid is then removed through the silica gel. The method of Stoltz et al. (U.S. Pat. No. 5,407,860) first deposits a non-wetting material over conducting leads and the exposed substrate surfaces between them. The nonwetting material is then selectively removed from the tops of the leads and the substrate surface, but is left on the sides of the leads. A dielectric material subsequently deposited over the leads does not enter the spaces between the leads, leaving them filled with air. Finally, the method of Rostoker et al. (U.S. Pat. No. 5,744,399) forms a porous dielectric gap-filling layer of low dielectric constant by using a composite material comprising a matrix-forming material and a fullerene. When the fullerene is subsequently removed from its matrix by dissolution, it leaves behind air-filled voids within the matrix.
The present invention is a method for introducing air into the gaps between adjacent conducting structures by extracting a disposable gap-filling material through a novel air-bridge structure. The formation of this structure as an integral part of the fabrication process and the subsequent manner of its use, makes the present invention different from the other inventions described above.